Recently, iron steel materials have gained use in diversified applications and have come to be frequently used in harsher environments than ever. Associated with this fact, demands on properties such as mechanical characteristics of steel products also have become severer than before. Under these circumstances, low-carbon high-manganese steel which possesses high strength and high workability has been developed for purposes of increasing strength of structural objects and reducing weight and cost thereof. The low-carbon high-manganese steel has been widely used in various fields such as steel sheets for line pipes and steel sheets for automobiles. Here, the “low-carbon high-manganese steel” refers to steel having a carbon concentration of 0.05 mass % or less and a manganese concentration of 0.5 mass % or more.
Cheap manganese sources include manganese ore or high-carbon ferromanganese, or the like, which are used in the steelmaking process to control the manganese concentration in molten steel. The smelting of low-carbon high-manganese steel involves throwing manganese ore as the manganese source into a converter; or adding high-carbon ferromanganese as the manganese source to molten steel being tapped from the converter, during decarburization refining of hot metal in the converter. Thus, the smelting increases the manganese concentration in molten steel to a predetermined concentration while cutting down cost associated with the manganese source (see, for example, Patent Literature 1).
However, in case of using these cheap manganese sources, reduction of the manganese ore leads to a failure to lower sufficiently the carbon concentration in molten steel through the decarburization refining in the converter, or the carbon present in high-carbon ferromanganese gives rise to an increase in carbon concentration in the molten steel that has been tapped. Thus, if there is a risk that the carbon concentration in molten steel will exceed the limit acceptable for low-carbon high-manganese steel, the molten steel that has been tapped needs to be further decarburized (refined).
As a known method for efficiently removing carbon from molten steel tapped from a converter, there is one decarburizing method which involves exposing the molten steel in a non-deoxidized state to a vacuum environment with use of vacuum degassing equipment such as an RH vacuum degassing apparatus; and decarburizing the steel by the reaction between dissolved oxygen contained in the molten steel (oxygen dissolved in the molten steel) and carbon in the molten steel. The alternative decarburization method involves blowing an oxygen source such as oxygen gas to molten steel under vacuum so as to oxidize carbon in the molten steel with the oxygen source thus supplied.
These decarburization methods under vacuum are called the “vacuum decarburization refining” in contrast to converter decarburization refining which takes place under atmospheric pressure. To remove carbon traced to a cheap manganese source by vacuum decarburization refining, for example, Patent Literature 2 proposes a method in which high-carbon ferromanganese is added into molten steel at an initial stage of vacuum decarburization refining in vacuum degassing equipment. Further, Patent Literature 3 proposes a method wherein high-carbon ferromanganese is added during the smelting of ultralow-carbon steel in vacuum degassing equipment, the addition taking place by the time when 20% of the vacuum decarburization refining time passes. In the vacuum decarburization refining of molten steel containing a large amount of manganese, however, oxygen reacts not only with carbon in the molten steel but also with manganese in the molten steel, with the result that the manganese added is lost by oxidation and the manganese yield is decreased. Further, the reaction makes it difficult to control the manganese content in the molten steel with good accuracy.
Regarding the oxygen source and the approach to promoting decarburization reaction in vacuum decarburization refining, for example, Patent Literature 4 proposes a method in which solid oxygen such as mill scale is added into a vacuum vessel to allow decarburization reaction to occur preferentially while suppressing the oxidation of manganese. Patent Literature 5 proposes a method wherein molten steel is refined by vacuum decarburization in such a manner that the converter blowing is terminated at a controlled carbon concentration in the molten steel and at a controlled temperature of the molten steel, and manganese ore is added to such molten steel in a vacuum degassing apparatus.
Patent Literatures 6 and 7 propose methods wherein molten steel tapped from a converter is refined by vacuum decarburization with an RH vacuum degassing apparatus in such a manner that a MnO powder or a manganese ore powder is top-blown together with a carrier gas toward the surface of the molten steel in a vacuum vessel. Patent Literature 8 proposes a vacuum decarburization refining method wherein a manganese ore powder is blown into molten steel in a vacuum vessel of an RH vacuum degassing apparatus together with a carrier gas through nozzles disposed on the sidewall of the vacuum vessel to decarburize the molten steel by means of oxygen in the manganese ore and also to increase the manganese concentration in the molten steel.
Meanwhile, there have been increasing demands for enhanced material characteristics in association with the increase in added values and the widening of applications of iron and steel materials. One approach to meeting such demands is to increase the purity of steel, specifically, to desulfurize molten steel to an ultralow level.
The smelting of low-sulfur steel generally performs desulfurization at a hot metal stage where the desulfurization reaction attains high efficiency. However, it is difficult for the desulfurization at the hot metal stage alone to attain sufficient reduction in sulfur concentration to the desired content of 0.0024 mass % or less for low-sulfur steel or 0.0010 mass % or less for ultralow-sulfur steel. Thus, the manufacturing of low-sulfur steel with a sulfur content of 0.0024 mass % or less or ultralow-sulfur steel with a sulfur content 0.0010 mass % or less involves desulfurization not only at the hot metal stage but also after the molten steel has been tapped from the converter.
Numerous methods have been heretofore proposed for the desulfurization of molten steel tapped from a converter, with examples including injection of a desulfurization agent to molten steel in a ladle, and addition of a desulfurization agent to molten steel in a ladle followed by stirring of the molten steel and the desulfurization agent. These methods, however, add a new step (a desulfurization step) between the tapping of steel from a converter and the treatment in vacuum degassing equipment, and thus cause problems such as temperature drop of molten steel, increase in production costs, and decrease in productivity.
To solve these problems, attempts have been made in which a desulfurization function is incorporated into vacuum degassing equipment to bring together and simplify secondary refining steps. For example, Patent Literature 9 proposes a method for the desulfurization of molten steel using vacuum degassing equipment wherein molten steel is introduced into a vacuum vessel of an RH vacuum degassing apparatus equipped with a top blowing lance, and a CaO-based desulfurization agent is thrown (blown) together with a carrier gas from the top blowing lance onto the bath surface to desulfurize the molten steel.
When, however, an oxide powder such as manganese ore for smelting low-carbon high-manganese steel or a CaO-based desulfurization agent for desulfurization is thrown from a top blowing lance during refining in vacuum degassing equipment, the temperature of molten steel is decreased by the sensible heat and latent heat of the oxide powder that is thrown or by the decomposition heat required for thermal decomposition. Such a temperature drop of molten steel is compensated for by an approach such as to increase beforehand the molten steel temperature in a step upstream of the vacuum degassing equipment, or to add metallic aluminum to the molten steel during refining in the vacuum degassing equipment to use the combustion heat of aluminum to raise the molten steel temperature. However, the approach which involves increasing of the molten steel temperature in a step upstream of the vacuum degassing equipment is accompanied by significant wear and damage of refractory materials in the preceding step, and brings about an increase in cost. The approach to increasing the temperature by the addition of metallic aluminum in the vacuum degassing equipment is disadvantageous in that, for example, the cleanliness of molten steel is deteriorated due to the resulting aluminum oxide, and the cost of auxiliary materials is increased.
Methods have been then proposed which involves throwing an oxide powder while suppressing a temperature drop of molten steel. For example, Patent Literature 10 proposes a method in which an oxide powder such as manganese ore is thrown onto the bath surface of molten steel while being heated by a flame of a burner disposed at the leading end of a top blowing lance. Further, Patent Literatures 11 and 12 propose methods in which molten steel is desulfurized with a CaO-based desulfurization agent thrown from a top blowing lance in such a manner that oxygen gas and combusting gas are jetted together from the top blowing lance so as to form a flame at the leading end of the top blowing lance, and the CaO-based desulfurization agent, after being heated and melted with the flame, is delivered to the bath surface of the molten steel.
The above refining methods have an object of enhancing the reaction rate and increasing the temperature of molten steel by heating powders such as manganese ore and a CaO-based desulfurization agent with a flame formed at the leading end of the top blowing lance in the vacuum degassing equipment, the powders heated being thus delivered to the molten steel. In this type of a refining method, the dynamic pressure of the jet flow ejected from the top blowing lance affects not only the yield of manganese ore and the desulfurization efficiency of the CaO-based desulfurization agent, but also affects the efficiency of heat transfer mediated by the powders. That is, if the jet flow is ejected from the top blowing lance without appropriate controlling of its dynamic pressure, the effect of the flame cannot be taken advantage of sufficiently. However, the conventional techniques including those described in Patent Literatures 10, 11 and 12 do not specify the dynamic pressure with which the jet flow is to be ejected from the top blowing lance.